Full support for ssh certificates

The main change in this release is the introduction of proper support for SSH certificates, contributed by @calderonth. Thank you so much for adding this feature!

The public api has received some minor changes. The verify() method now accepts any implementation of Into<Identity> instead of &PublicKey. As this trait has been implemented for &PublicKey, porting existing code should be trivial. However, if the previous concrete type was used for type inference, you might need to add an explicit type.